1. Field of the Invention
The present invention relates to the field of pipe connections. More particularly, the present invention relates to the field of welded pipe connections normally useful in the oil and gas production, and refining and transportation industries, as well as flanged pipe connections normally useful in the chemical plant industry.
2. Background of the Related Art
Tubular goods, such as pipe used to transport oil and gas and products thereof, must be capable of withstanding the corrosive and/or erosive attributes of materials passing therethrough without failure. Such pipe is commonly manufactured from alloy steels which have insufficient anti-corrosive and/or anti-erosive properties to withstand attack from the gasses and liquids which are passed therethrough. Therefore, the interior regions of these pipes are commonly coated with protective materials, such as thin polymer based coatings or cement based liners, which form a protective barrier between the pipe material and the materials passing through the pipe. Where the pipe may be continually exposed to highly corrosive environments, such as in chemical plants where hydrofluoric or hydrochloric acid might be flowed through the pipe, the protective barrier may be constructed of a tubular PTFE based material, such as a PTFE based material sold by DuPont Corporation Tefzel®, or with other erosion or corrosion resistant materials in tubular form that extend the length of the interior diameter of the pipe.
Pipe used to transport oil, gas, and their products is typically configured in lengths of up to approximately 60 feet, and more typically at lengths of less than 45 feet. Therefore, to span any substantial distance using this pipe, the individual lengths of pipe must be connected end to end. In the oil and gas transportation industry, the most common method of connecting the individual lengths of pipe is by welding their ends together. Welding of the pipe ends presents several pipe material protection problems. First, where the pipe is protected by a thin polymer coating, the heat generated during welding destroys the coating adjacent the weld joint. This exposes the pipe material, and the weld, to the corrosive and erosive fluids passing through the pipe. Therefore, the weld area, and the pipe adjacent the weld, must be supplementally protected from the erosive and/or corrosive pipe environment.
One method of protecting the weld connection from the material flowing through the pipe is to apply a protective coating to the interior of the weld connection after welding. After several pipe lengths have been welded together, a re-coating pig is sent down the pipe to re-coat the weld joint in-situ. This in-situ re-coating is expensive and time consuming.
Another method of protecting the area of the pipe adjacent a weld employs an intermediate insert which fits into the pipe adjacent a weld joint to form a physical barrier between the weld and the materials passing through the pipe. One such insert is shown in U.S. Pat. No. 5,219,187, by Mikitka, wherein the insert is configured as an internally coated coupling provided in a supplemental pipe segment, which supplemental pipe segment is welded to one end of a pipe. The coupling is integrally provided in the pipe segment, preferably covers the entire inner diameter of the pipe segment, and also extends outwardly from the free end of the pipe segment when the pipe segment is welded to the pipe. To connect the length of pipe with the coupling projecting therefrom into an adjacent pipe, the coupling is inserted into the end of the adjacent pipe, and the free end of the pipe segment is welded to the end of the adjacent pipe. When the pipe segment and adjacent pipe end are welded together, a portion of the protective coatings on the interior of the pipe and coupling are destroyed by the heat of the weld. Additionally, if the coupling is damaged at any point, the entire pipe to which it is attached is rendered useless.
Another insert for protecting pipe ends at weld joints is disclosed in U.S. Pat. No. 4,913,465, by Abbema. In that reference, a metallic coupling is placed into the ends of two adjacent pipes prior to welding the adjacent ends of the pipe together. The coupling includes a circumferential recessed area, which aligns under the weld as the weld is formed, and a seal disposed on either side of the recessed area. An insulative wrap and a plurality of heat retaining strips are received in the recessed area. The heat retaining strips span the recessed area and contact the mass of the metallic coupling at either end of the strip. Each strip also includes alignment bosses thereon, to which the pipe ends are physically engaged to provide a pre-selected gap between adjacent pipe ends and to center the coupling within the two pipe ends. These alignment bosses are sacrificed into the weld during welding.
The connection system disclosed in Abbema has several limitations. First, the coupling is metallic and therefore transfers a substantial amount of heat from the welding operation along the inner diameter of the pipe. This heat can destroy the interior protective coating on the pipe at a substantial distance inwardly of the pipe end. In an attempt to mask the area of the pipe where the protective layer is destroyed, the coupling is configured as a spanning element, i.e., it spans the burned or otherwise destroyed portion of the interior pipe coating adjacent the pipe ends. Additionally, the coupling itself is also subject to corrosion or erosion when exposed to the pipe liquids or gasses. In an attempt to obviate any corrosion or erosion problem with the coupling, a secondary protective coating is applied, before the coupling is inserted into the pipe ends, to the inner diameter of the coupling and to the portion of the outer diameter of the coupling adjacent the ends of the coupling. Also, a mastic is applied to the inner diameter of the pipe. The mastic lubricates the coupling upon insertion of the coupling into the pipe end and provides a secondary coating barrier if the coating on the outer diameter of the coupling is damaged. However, during welding operations, the heat of welding will travel through the heat retaining straps and into the coupling at discrete spots around the circumference of the coupling, and this heat will transfer through the coupling and create localized burned areas of protective coating at the inner diameter of the coupling. The mastic will also be partially destroyed by heat during welding operations, and the mastic may become disengaged from the coupling ends and expose any defects in the coupling coating to the erosive and corrosive pipe environment. Further, the seal configuration on the coupling does not fully protect the weld area from the erosive and/or corrosive conditions within the pipe. The seal provided on either side of the recess cannot span the possible gaps which may be present as a result of the tolerance on the pipe inner diameter. Therefore, when the pipe inner diameter is at the high end of the acceptable tolerance, the seal may not engage the pipe. Likewise, when the pipe inner diameter is at the low side of the tolerance, the seal may be destroyed as the coupling is shoved into the pipe end, particularly if the seal is configured for the high end of the inner diameter tolerance. In either case, fluids passing through the pipe may enter the annular area between the coupling and the pipe. Additionally, the mastic may interfere with the seating of the seals against the inner diameter of the pipe, which will allow pipe liquids and gasses to leach between the coupling and the pipe. Finally, the bosses used to align the pipe ends and maintain the proper weld gap may, when sacrificially incorporated into the weld, reduce the strength of the weld and thereby reduce the effectiveness of the weld connection.
The corrosive nature of some fluids also limits the utility of pipelines or runs wherein the individual pipe segments are welded together. Pipelines and pipe runs used in chemical plant applications also have erosive and/or corrosive fluids passing therethrough, but are typically constructed differently than as described above for welded pipe connection. Pipe used in chemical plant applications, although readily available in lengths of up to 40 feet, is typically configured in lengths of only 10 feet, and occasionally in lengths of up to 20 feet. The limiting factor on pipe length in chemical plant applications is the need to provide a barrier between the steel or other material forming the pipe, and the potentially corrosive or erosive materials flowed through the pipe. Standard industry practice is to provide this barrier by pulling a length of protective tubing, such as the above-mentioned Tefzel® material, through the pipe segments to form a barrier between the pipe material and the material flowed through the pipe. The pipe ends cannot be welded where such an inner barrier material is used, because the heat of welding the pipe will destroy the barrier material, and there is no convenient means for connecting the lengths of protective barrier material tubing extending within the pipe that is capable of withstanding the forces generated within the barrier material as materials are flowed therethrough. Therefore, to connect adjacent lengths of this pipe, the individual pipe ends are provided with flanges, and the end of the tubular barrier material within each length of pipe is flared outwardly to be received between the flanges. By connecting adjacent pipe flanges, the ends of each segment of the tubular barrier are secured between the flanges, and a continuous barrier having a circumferential joint at the flanges is provided.
The flange method of joining adjacent lengths of pipe, and the inner barrier material, is expensive, time consuming, and subject to failure. One primary failure mode which occurs with this connection system is a stress fracture in the barrier material where the barrier material is flanged outwardly to be received between the flanges of the adjacent pipe ends. Because the barrier material typically has a higher coefficient of thermal expansion than the pipe material, the barrier material expands and contracts as the pipe thermally cycles in use. As the tubular barrier is fixed only at its ends, i.e., at the flanges, the tubular barrier has some freedom to move except at the flanges, and thus the stress caused by thermal expansion of the barrier material is highest where the tubular barrier is flared outwardly to be joined in the flange. Thus, the tubular barrier will crack at this location, necessitating removal of the pipe and replacement of the tubular barrier material. This commonly requires disassembly of a substantial length of the pipe line extending from the failure point to an elbow, or other location in the pipe run.
The second major problem associated with the interconnection of the tubular barrier material at a flange connection also relates to the higher coefficient of thermal expansion of the tubular barrier as compared to the pipe material. The longer the length of the tubular barrier, the greater the total linear expansion or contraction of the tubular barrier over a given temperature range. Pipe lengths in the chemical processing industry are generally limited to 20 foot lengths, because longer lengths would create excessive thermal expansion and cause the tubular barrier to break at the aforementioned flange position or to buckle in the pipe.
A third problem associated with the connection of the tubular barrier material between the pipe flanges is the difficulty of forming the connection in all seasons and environments. The tubular barrier material has a memory and tends to return to its final shape after being flared to be received in the flange, which return to the initial configuration occurs fastest at high temperatures. At low temperatures, the formability of the material is low, so the time needed to flare the tubing is increased, and the brittleness is greater, so the chance of breaking the tubing while forming the flare is increased. These factors add up to provide a connection that is difficult to form.
A fourth problem associated with flanged pipe connection is material fabrication and availability. Flanged pipe is not readily available in different pipe lengths for all pipe diameters, and the pipe line or pipe run fabricator typically has to weld flanges onto the pipe on site, or special order flanged pipe of various lengths, to provide the major runs of pipe on the job site. In either case, the flanged pipe is more expensive to provide for a given pipe line or pipe run, than a welded pipe line or pipe run.